In recent years, a thin film transistor including a thin semiconductor film (with a thickness of several nanometers to several hundreds nanometers, approximately) over a substrate having an insulating surface (e.g., a glass substrate) has been attracting attention. Thin film transistors are widely used for ICs (integrated circuits) and electronic devices such as electro-optical devices. In particular, thin film transistors are rapidly developed as switching elements of image display devices typified by liquid crystal display devices and the like. In image display devices such as liquid crystal display devices, a thin film transistor using an amorphous semiconductor film or a thin film transistor using a polycrystalline semiconductor film is mainly used as a switching element. Further, a thin film transistor using a microcrystalline semiconductor film is known (Patent Document 1, for example).
The performance of a thin film transistor is more excellent as the amount of change in current which flows between a source and a drain is larger when gate voltage is changed. A subthreshold swing is widely known, which shows the amount of change in current which flows between a source and a drain when gate voltage is changed.
In addition, in a thin film transistor, the amount of light leakage current needs to be reduced. The light leakage current is a current which is generated when the photovoltaic effect is caused in a semiconductor layer of the thin film transistor by irradiation of the semiconductor layer with light and flows between a source and a drain. Therefore, a lot of researches have been conducted on a technique for shielding the semiconductor layer of the thin film transistor from light (Patent Documents 2 and 3, for example).